1. Field of the Invention
Synthetic resins made from condensation products of polyhydric alcohols and polycarboxylic acids mixed with other materials to increase neutron shielding properties.
2. Description of the Prior Art
In the August, 1962 issue of Nuclear Engineering, pages 305-308, shielding materials for radiation are discussed including polyethylene and boron compounds. It is also known that polyethylene is one of the most effective plastic neutron shielding materials, if not the most effective. In tests against a commercial polyester resin (AROPOL WEP-41), a laminated hardboard (Benelex 70) and an acrylic resin (Plexiglass C), polyethylene was shown to be appreciably more effective than all in neutron shielding, being about 35 to 40% more effective than the polyester, i.e., about 35 to 40% more thickness of the polyester would be required to do the same neutron shielding job as polyethylene.
The polyester resin compositions of this invention having polyethylene incorporated therein have some significant advantages over polyethylene alone. For making large slabs or large complex shapes of polyethylene alone, large high pressure heated molding presses are necessary; whereas, for the polyester compositions of this invention casting large slabs or large complex shapes no heating and no pressure is required. Furthermore, since the polyester compositions of this invention are cross-linked they will not drip and flow in a fire situation as would polyethylene alone.
Polyethylene is an excellent shield for neutrons due to its high hydrogen content. Fast neutrons are slowed by repeated inelastic collision with hydrogen nuclei. A neutron can give up a significant proportion of its energy in a single collision with hydrogen, whereas collisions with nuclei of higher mass number result in much smaller energy transfer from the neutron to the nucleus. Thus hydrogen (and thereby polyethylene) is very effective in reducing the speed (or energy) of a neutron from its initial value to a value approaching that due to thermal motion.
Fast neutrons have a low probability of capture by a nucleus, the absorption cross-sections for fast neutrons being on the order of 30 microbarns.
Slow or thermal neutrons have a probability of capture which varies widely from isotope to isotope. Examples of the thermal-neutron capture cross-sections (cross-section is a measure of probability) for various isotopes are given below.
______________________________________ .sup.1 ll 0.33 barns 2.23 Mev .sup.6 Li 950 " none .sup.10 B 3840 " 0.478 Mev .sup.12 C 0.0034 " 4.95 Mev .sup.113 Cd 20,000 " 9.05 Mev .sup.151 Sm 15,000 " 7.9 Mev ______________________________________
From the above, it is evident that a number of isotopes absorb neutrons more strongly than does .sup.1 H, and would thereby be effective shields against thermal neutrons. Unfortunately, upon capturing (absorbing) a neutron, most nuclei emit a gamma-ray of an energy characteristic to that type of nuclei. The energies of these "capture .gamma.'s" are also indicated in the above table.
The capture .gamma. radiation is in itself a potential hazard to humans. The advantage of adding boron to an essentially hydrogen (polyethylene) shield is that most neutrons will be captured in the boron (probability of capture in boron is 10,000 times that in hydrogen), thereby reducing the energy of the capture gamma ray from the 2.23 Mev characteristic of hydrogen to the 0.5 Mev characteristic of boron-10. The gamma dose rate is thus reduced by a factor of 0.5/2.23 = 1/41/2.
Lithium-6 can be used in place of or in conjunction with boron-10 for thermal-neutron capture, and although the lithium does not have the efficiency for neutron capture that boron does, it does have the advantage of no capture .gamma. emission. Cadmium-113 is one of the most efficient of the common metals for thermal neutron capture, however, high energy capture .gamma. emission is generated and must be handled.
Only certain isotopes of the elements are effective in radiation shielding, but hydrogen, boron, lithium and cadmium in their natural state have adequate quantities of the desired isotopes, and processing to enrich the amount of desired isotope is not necessary or desirable from an economic viewpoint.